Single mode semiconductor lasers with emitting apertures on the order of 2 um×2 um are generally limited to roughly a watt of output power. Higher power lasers can be realized by increasing the output aperture much wider to a 100 um or so. The laser becomes multimode in the wide lateral direction, but stays single mode in the vertical direction. Such lasers can provide on the order of 5 to 10 Watts of optical power reliability and are used in various applications such as materials processing and industrial processes.
In addition to the total power output, another key performance metric of these lasers is the “brightness”, which is inversely proportional to the product of the emitting area and the numeric aperture—the brighter the source, the smaller the aperture or the smaller the divergence of the beam. A brighter source is generally preferred as it can be focused tighter to provide higher energy density.
A recent commercially interesting application of these broad area lasers is for pumping double clad fiber lasers. Generally multiple broad area lasers are individually coupled to multimode fibers. These fibers are in turn combined into even larger core diameter multimode fibers, which subsequently pump the double clad fiber. Hundreds of watts of pump power can be obtained in this manner. However, since “brightness” must be preserved, the more power one desires, the larger the core diameter or the NA of the final merged fiber. For example, if one desires 90 Watts of pump power using nine elements of low individual sources that are coupled to 100 um core diameter fibers, the final fiber in the assembly should have an area that is 9 times as much or a diameter of 300 um microns. As the diameter of the final combined fiber increases, the overlap with the single mode core of the double clad fiber decreases. This large core size in turn lowers efficiency and makes the fiber harder to manufacture and handle. Thus it is usually desirable to use the brightest sources possible in the first place.
Considerable brightness is wasted in the first step of the process, where a 100 micron broad area laser with a vertical aperture of 2-3 um is coupled to a 100 micron diameter round fiber. Though the laser fills the fiber laterally, vertically a lot of the fiber area is wasted. In theory multiple lasers of such kind can be coupled to a single multimode core if appropriate packaging can be realized.